The Swedish Open gets underway on July 14, less than a fortnight after Williams retired from her Wimbledon doubles match with sister Venus. Williams looked disoriented and listless as she withdrew in the first set on Tuesday, but tournament director Nina Wennerstrom told ESPN that the World No.1 has entered the tournament in Bastad and plans to take part.

Williams, who lost in the third round of singles the previous Saturday, looked out of sorts from the moment the pre-match warm-up began on Court No.1. It was an odd scene, as the 32-year-old had trouble collecting balls from the ball boys and girls and struggled to hit practice strokes.

The start of the match was put off for about 10 minutes while Serena sat in her sideline chair and was examined by medical staff, including a check on her blood pressure. At one point, Serena hunched over and covered her face with her hands.

"I am heartbroken I'm not able to continue in the tournament," Serena said in a statement. "I thought I could rally this morning, because I really wanted to compete, but this bug just got the best of me."

Serena and Venus have won 13 Grand Slam titles together, including five at Wimbledon. In singles, Serena has won 17 major titles, the most of any active woman.